The centromere is a specialized region of the eukaryotic chromosome. It is the site of kinetochore formation, a structure which allows the precise segregation of chromosomes during cell division. In addition to this, a possible structural role in the higher-order organization of eukaryotic chromosomes has also been suggested (Hadlaczky (1985), Internatl. Rev., 94:57-76).
The isolation and cloning of centromeres is crucial, not only to understanding their molecular structure and function, but also for the construction of stable artificial chromosomes. Taking advantage of the existence of centromere-linked genes, functional centromeres of lower eukaryotes (yeast) have been successfully isolated (Blackburn, et al. (1984) Ann. Rev. Biochem., 53:163-194; Clarke, et al. (1985), Ann. Rev. Genet., 19:29-56). The combination of a functional centromere with telomeres, which stabilize the chromosome ends, permitted the construction of yeast artificial chromosomes (Murray, et al. (1983) Nature, 305:189-193; Burke, et al. (1987), Science, 236:806-812). This initiated a new era in the study of chromosome function and in genetic manipulation.
Higher eukaryotes (e.g., mammals), in contrast to yeast, contain repetitive DNA sequences which form a boundary at both sides of the centromere. This highly repetitive DNA interacting with certain proteins, especially in animal chromosomes, creates a genetically inactive zone (heterochromatin) around the centromere. This pericentric heterochromatin keeps any selectable marker gene at a considerable distance, and thus repetitive DNA prevents the isolation of centromeric sequences by chromosome "walking."
Thus there is a need in the art for mammalian artificial chromosomes. Use of such chromosomes could overcome problems inherent in present techniques for introduction of genes into mammalian cells, including the concomitant creation of insertional mutations, size limitations on introduced DNA, and imperfect segregation of plasmid vectors.